Gallium nitride (GaN) is commonly cited as a superior material for high-voltage power devices due to its wide bandgap and associated high electric field required for avalanche breakdown. Ideal bulk GaN crystals have critical fields in excess of 3,000,000 V per centimeter. However, in practice, a high electric field needed for avalanche breakdown is lowered by non-idealities that are present within the structure of a GaN device. During high voltage operation of a GaN device, electrical breakdown will typically occur at defects and/or at locations with a concentrated electric field. An example of such a breakdown location is a corner of a Schottky gate. An ideal structure comprising a bulk crystal such as silicon carbide (SiC) or GaN will avalanche uniformly in a high electric field region. As a result, avalanche energy is distributed uniformly, which greatly enhances the survivability of a device made up of an ideal bulk crystal. For example, vertical p-n junctions fabricated in SiC homoepitaxial layers demonstrate avalanche breakdown ruggedness. However, breakdown in defective GaN layers will typically occur at defects within defective GaN layers. A resulting high energy density typically causes irreversible damage to a device that includes defective GaN layers.
Another factor impacting breakdown ruggedness is the nature of the metal semiconductor contacts that carry a breakdown current. Previous work with SiC Schottky diodes has demonstrated that Schottky contacts can be degraded by avalanche current. In response to this problem, junction barrier Schottky diodes have been developed to urge avalanche breakdown to occur across a bulk p-n junction with ohmic contacts rather than Schottky contacts. Thus, the breakdown ruggedness of GaN high electron mobility transistors (HEMTs) may be limited by breakdown events in highly localized areas within a semiconductor due to crystal defects and/or electric field concentration. Moreover, the breakdown ruggedness of GaN HEMTs may be limited by an electrical breakdown of adjacent dielectric layers and/or high current flow through the Schottky gate electrode during breakdown events. Thus, there is a need to provide overvoltage protection for a GaN device to ensure that the GaN device handles a typically destructive breakdown voltage without being damaged.